The act of masturbating in a locker of anykind (P.E, School, Gym, Etc.) It is done by putting your genitalia in your locker and masturbating with majorly attracting a disgusted crowd. It is usually done with a third party tool such as a dildo or a pocket pussy. Or you can go old school with lube and your dominant hand.

*from afar*
Matt: Why is Griffin so close to his locker? It looks like he's trying to pick up a heavy textbook.
Gavin: He's pulling a locker fap.
Matt: I thought he was just making funny faces because he's dicking around.